We will continue studies on the flavoproteins NADPH-cytochrome c reductase of liver microsomes, NADPH-adrenodoxin reductase, Electron Transfer Flavoprotein (ETF) of liver mitochondria, and bacterial salicylate hydroxylase. We will initiate studies on the nitrite reductase of spinach, where we just demonstrated that the prosthetic group is the same as the E. coli sulfite reductase heme, siroheme. Our objectives are to examine these enzymes and to describe the mechanism whereby their prosthetic groups participate in electron transport.